According to industry analytics, searches for pregnancy-related adult content have shot up by nearly 20 percent since 2014.

Pregnant women often say they get a lot of smiles. They’ll talk about people opening doors for them, or offering them a seat on a crowded subway. They’ll talk about friends and relatives patting their belly. It is decidedly less often they’ll talk about being propositioned for sex. But, of course, it does happen.

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It’s no secret that some men harbor a fetish for pregnant women. According to porn industry analytics, searches for pregnancy-related content have shot up by nearly 20 percent since 2014; and the term “pregnant” is now the 107th most popular porn search in the United States, putting it right up there with “redhead” and “babysitter.” Unsurprisingly, the NSFW corners of Reddit are filled with thousands of posts containing erotic images and ideas relating to women who are expecting.

The pregnant form has been worshipped by societies for millennia, be it via fertility goddesses or simply treating women who are expecting with the adoration often reserved for kings and queens. But that was mainly due to the fact that women can carry and deliver children, and thus prolong the bloodline. What is it that drives such fervent fetishization of pregnant women in society today?

“With some fetishes, we believe there’s an imprinting process,” explains Michael Aaron, a NYC-based sex therapist who specializes in alternative sexualities. “Something very impressionable may have happened when they were young, and they eroticized it.”

In 2010 study that was later published in the Journal of Sexual Medicine, a team of researchers found that early exposure to pregnancy and lactation can lead to an adult interest in pregnant women down the line. Older siblings, researchers found, are more likely to develop this kind of attraction than those who never witnessed the birth of a baby brother or sister.

In his book Modern Sexuality, Aaron explains that testosterone has been shown to influence a predisposition to fetishistic interests, which is why members of the pregnancy fetish community appear to be overwhelmingly male. “We don’t have much research on it, but most accounts indicate that men have much higher prevalence of object-oriented fetishes,” he explains. “That tends to come out as their sexuality develops. Men may be more likely to fetishize a round belly, or larger breasts heavy with milk.”

As it happens, lactation has been so frequently eroticized that adult breastfeeding have become somewhat of a staple in the community. Hell, this kind of kink has proved so popular that even certain devout Christians are getting into it.

“There are different aspects to the fetish just like there are different aspects to pregnancy,” explains Aaron. “You can have 10 different people in a room doing the same thing for 10 different reasons.”

Some people simply enjoy embracing the taboo Aaron explains, as such might make it more erotic. “Here you are having hot sex with someone who is preparing for motherhood,” he says. “It’s sort of like trying to have sex with a nun.”

Dating Pregnant Women (dot com), a personal site that connects interested individuals to pregnant women in their area, describes the adoration of expecting women as such: “There is something about pregnant women that is just so beautiful. Women look their best when they are carrying a child. Their skin is flawless, they have a happy sparkle in their eyes and the bigger they are, the better.”

And this hints at another dynamic at play, one that’s not nearly as kinky as the fetishists would have you think: Blood flow increases during pregnancy, giving some women that special “glow.” The influx of hormones can give you thicker nails and shiny hair, making them more attractive to the opposite sex. It’s also an obvious sign of fertility, something that strikes men looking to procreate. One study published in the Journal of Psychology and Human Sexuality found that men desire their partners more so during pregnancy than ever before.

But this, of course, isn’t all about catering to the male gaze. The hormonal surge women experience during pregnancy can also make them horny as hell, making things like pregnancy porn and sex while expecting especially hot.

Indeed, those who fall into the niche community informally known as pregnancy fetishists want to have sex with women because they’re pregnant. But it’s worth noting that many more simply want to have sex with women because, well, they simply want to have sex with women, pregnant or not.